Traditionally the use of calcic or pure sodic bentonite is known in the composition of green sand for use in metal casting, where its function is to confer mechanical properties to the mold, especially a high plasticity and a low friability, thus avoiding occurrences of mold breaking or their erosion during the metal pouring process.
It is a fact that along the last years and as a result of the exacerbate use of natural resources, sodic bentonite, be it natural or activated, does not offer a reasonable purity level anymore, and this factor is interfering with the quality of green sand used for elaboration of foundry molds and consequently with the quality of the obtained castings. For this reason, the additivation of the bentonite has become necessary in order to achieve better technical results and to improve production, aiming at minimizing and even reducing such inconveniences.
Just for the purpose of better understanding in relation to the state-of-the-art, the green sand, an indispensable composition for use in casting, is composed of the mixture of variable portions of silica sand, clay, known as bentonite, water and mineral coal powder, also known as Cardiff powder.
This compound has been known for many years in foundry and used in conventional casting as well as in high productivity molding machines and, for this purpose, it needs a sufficiently high compactability in order to assure an excellent plasticity of the molds, thus avoiding their breakage resulting from excessive heat of the molten metal and during the mold extraction.
The present invention provides a composition of sodium bentonite, obtained through addition of starch and sugar additives or dextrin and sugar, specifically during the transformation process of calcium bentonite into sodium bentonite, together with the incorporation of sodium carbonate into the natural ore.
Therefore, the green sand must not present a high moisture level, and the biggest problem currently faced for obtaining a green sand having both high compactability as well as low moisture is the fact that such characteristics are directly proportional, and in most cases their adjustment cannot be achieved by simply modifying the addition of one of the mixture components.
Thus, there are several alternatives to be adopted by foundries for obtaining a green sand composition having good compactability and moisture rates, and one of those alternatives is by means of using additivated bentonite which is the subject matter of the present patent.
It is important to explain that the sodic bentonite, resulting from the activation through sodium carbonate, must present a high thermal stability, high swelling level and the highest possible wet tensile strength and green compression strength of the molding sand, such characteristics resulting from the fact that the ore presents a high methylene blue adsorption rate.
Now it has turned out that the Brazilian layers which contained such ore with high methylene blue adsorption have virtually run out and, due to this fact, it will be necessary to search for efficient alternatives as substituting elements in the composition of green sand in order to assure the plasticity of casting molds.